Fix a bug where redactions were not being sent over federation if we did not have the original event.

[H-Shay]

Prior to this change, redactions were not sent over federation if the sending server hadn't received the original event. Fixes #12795. Per the discussion on #12795, this is achieved by making the function EventsWorkerStore._get_events_from_cache_or_db public and using that rather than EventsWorkerStore.get_events_as_list, which drops redaction events if the original event isn't present. I wrote a test to verify that this change works, you can see it here: https://github.com/matrix-org/complement/actions/runs/3055371487/jobs/4929450057

[DMRobertson]

Complement test is https://github.com/matrix-org/complement/pull/462

[richvdh]

We now have get_events_from_cache_or_db and get_events_as_list which both sound like they do the same thing but have different behaviour with regard to redaction events.

I don't disagree, and it's basically the point I made on #12795:

having lots of similar-but-different entry points makes a component hard to use correctly.

Still, I can't think of a better solution here. What we could do is try to find names that better describe the functionality (maybe s/get_events_from_cache_or_db/get_unredacted_events? other suggestions welcome) and spell out the differences between the entry points in the docstrings.

[H-Shay]

Merged #13813 into develop, thanks for the explanations and the review!